1. Field of the Invention
The present invention relates to a control method of injection molding and a control apparatus of injection molding suitable for the use of the control method.
2. Background Art
An injection molding machine generally includes a molding unit, a clamping unit, and an injection unit, and the molding unit has a stationary die and a movable die. Mold closing, mold clamping, and mold opening of the molding unit are performed by allowing the movable die to move forward and backward using the clamping unit. A cavity space is defined between the stationary die and the movable die in association with the mold clamping. The injection unit has a heating cylinder and a screw provided to be rotatable and movable forward and backward inside the heating cylinder. It also has a metering motor and an injection motor for allowing the screw to rotate and move forward and backward.
In the metering process, resin is forced forward by rotating the screw and stored ahead of the screw inside the heating cylinder. In the injection process, the stored resin is injected from an injection nozzle provided at the front end of the heating cylinder by allowing the screw to move forward. The resin thus flows through a runner inside the molding unit and enters into the cavity space via a gate, so that it is filled in the cavity space. By cooling the molding unit thereafter, the resin inside the cavity space is cooled and solidified to consequently form a molded article.
A control method and a control apparatus of injection molding of this type in the related art are described, for example, in JP-A-2001-277322 (hereinafter, referred to as patent document 1). The patent document 1 describes a filling process control method and a control apparatus for an injection molding machine. According to the filling process control method for an injection molding machine of the patent document 1, when the screw has moved forward until it reaches a predetermined position in the filling process of injection molding, the screw is returned to a set position at a set velocity for a necessary pressure wave to be formed by means of depressurization.
According to the invention of the patent document 1 (hereinafter, referred to as the first related art), when the screw has moved forward until it reaches the predetermined filling position (set value), the screw is moved backward to the set position at the set velocity. Accordingly, because the screw can be operated in response to the velocity control, abrupt depressurization is enabled, which makes it possible to set a necessary pressure waveform as desired. It is therefore expected to achieve an advantage that the quality of a molded article can be stabilized (see Paragraph [0030] of the patent document 1).
Another example of the injection molding machine in the related art is described, for example, in JP-A-3-243321 (hereinafter, referred to as patent document 2). The patent document 2 describes a control method of an electric injection molding machine using a servo motor as the drive source for injection and holding pressure. The control method of the electric injection molding machine of the patent document 2 relates to a control method of an electric injection molding machine for switching the injection process and the holding pressure process in the injection apparatus using a servo motor as the drive source. According to this control method, a minor feedback of an injection velocity is provided to a holding pressure control system and this minor feedback is shared with a velocity feedback system of an injection velocity control system. An operation signal to the minor feedback of an injection velocity of the holding pressure control system during the injection process is compared with an injection velocity set signal, and either one of these two signals, whichever is the smaller, is selected and used as a velocity command signal.
According to the invention of the patent document 2 (hereinafter, referred to as the second related art), the continuity of an injection pressure when the control is switched from the injecting process to the holding pressure process is ensured to protect the die. It thus becomes possible to obtain a satisfactory molded article by preventing the occurrence of flash. Further, it is expected to achieve an advantage that accuracy of a molded article can be improved by prolonging the life of the electric injection molding machine (see the column of Advantages of the Invention in the patent document 2).
Incidentally, TV sets and mobile electric appliances in these days have been becoming thinner and extremely thin molded articles are increasing. In order to meet such an increase, a high-velocity injection molding machine is in widespread use so that a molding material that is fluidized by heating is spread into every corner of a space for molded article (cavity space) inside the die before it is cooled and solidified. With the high-velocity injection molding by this high-velocity injection molding machine, it is obvious that a pressure loss occurring inside the die increases exponentially from the viewpoint of flow dynamics. In addition, with the high-velocity injection, inertia of the injection apparatus is so large that it becomes difficult to control the screw velocity when the control is switched to the holding pressure process. This poses a problem that an overshoot of injection pressure occurs.
To overcome this problem, the first related art performs control to reduce a pressure by moving the screw backward temporarily at the time of V (velocity)-P (pressure) switching by the control by which the control is switched from the injection process to the holding pressure process. Because the screw is moved backward temporarily before the holding pressure control is started, there is a delay in the follow-up to the subsequent holding pressure control. This delay makes the holding pressure control difficult for an extremely thin molded article. Further, a temporal overshoot causes a variance in a molded article, which poses a problem that adverse influences are given to the life of the die.
In the second related art, in order to prevent an overshoot of pressure when the control is switched from the injection process to the holding pressure process, the minor feedback of an injection velocity is provided to the holding pressure system and a velocity at the time of switching is controlled by sharing the minor feedback with the velocity feedback system of the injection velocity control system. This configuration, however, consequently causes a pressure drop because the screw is decelerated before the filling in the injection process is completed. Accordingly, there is a problem that a short shot readily occurs in an extremely thin molded article or a thin portion at the end of filling.
The problems discussed above will now be described more concretely in the following.
FIG. 1 is a view showing a concrete example of a molded article having a thin portion. The molded article 1 includes a product portion 2, a runner portion 3, and a gate portion 4 connecting the product portion 2 and the runner portion 3. The product portion 2 is formed of a rectangular thin plate member and a rectangular recessed portion 5 is provided in one surface thereof. The bottom of the recessed portion 5 of the production portion 2 is a product thin portion that is made thinner than the other portions.
For the molded article 1 having such a thin portion, a filling work by high-velocity injection is necessary because the filling of a molten molding material has to be completed before it is cooled and solidified inside the die. FIG. 4 shows a case example of a velocity waveform, a pressure waveform, a screw position in the case of molding by a molding method and control in the related art. Referring to FIG. 4, a graph indicated by a thick solid line represents a detection pressure P1, a graph indicated by a thin solid line represents a detection screw position N1, and a graph indicated by a thick alternate long and short dash line represents a detection velocity V1. Further, the abscissa is used for a molding time in FIG. 4. It shows an elapse of 0.5 second from the start of molding. About 0.03 second from the start of molding is the control of the filling process and the controls shifts to the holding pressure process thereafter.
Referring to FIG. 4, the detection pressure P1 rises abruptly from the start of molding and reaches the peak at a time point S1 (about 0.05 second from the start), after which it drops abruptly and returns to the vicinity of zero (0) at a time point S2 (about 0.13 second from the start). It then rises slightly and shifts to a preset specific pressure at a time point S3 (about 0.18 second from the start) and holds this pressure thereafter. The detection screw position N1 starts moving forward from the start of molding and reaches the front end at a time point S4 (about 0.05 second from the start). It then changes to a backward movement and returns to about half the distance at a time point S5 (about 0.13 second from the start). It subsequently moves forward slightly and maintains this position. The detection velocity V1 rises from the start of molding and reaches the peak at a time point S6 (about 0.03 second from the start), after which it drops until it shifts further in a minus direction and changes to rise at a time point S7 (about 0.12 second from the start). Subsequently, it returns to almost the initial velocity (0) at a time point S8 (about 0.14 second from the start) and holds this stopped state.
In this manner, according to the related art shown in FIG. 4, when the screw moves forward (moves downward in FIG. 4), the detection velocity V1 rises (upward in FIG. 4) so as to respond to the set speed VS1 at the start of injection, and so does the detection pressure P1. For the detection pressure P1 to exceed the set pressure PS1 at a time point S11 before the detection velocity V1 reaches the set velocity VS1, the control apparatus outputs a deceleration control signal at the time point S6. However, because the injection apparatus has inertial energy, the pressure rises instantaneously to the peak pressure at the time point S1. A difference SR between the peak pressure and the set pressure PS1 in this instance represents an overshoot of pressure.
Also, in this example, the V (velocity)-P (pressure) switching is performed while the screw is moving forward as indicated at the time point S9 and the control is switched from the filling process to the holding pressure process. The screw, however, keeps moving forward up to the peak pressure at the time point S1 and reaches the end of forward movement at the time point S4. This phenomenon is referred to as over packing and occurs when a molding material in an amount exceeding the capacity of the cavity space (space for molded article) in the die is placed into the die. This phenomenon not only develops remaining stress in a molded article, but also causes a defective dimension, a dimensional variance, and flash. The control to lower the pressure is continued after the control is switched to the holding pressure process and the screw position is abruptly moved backward as indicated at a time point S10. Accordingly, because the detection pressure P1 drops to or below the holding pressure set value PT1 as at the time point S2, a behavior for depressurization is induced, after which the control to hold the detection pressure P1 at the holding pressure set value PT1 is performed.
In the molding of a thin product as in the case example described above, the control method in the related art causes an overshoot of pressure during the filling and depressurization occurs after the control is switched to the holding pressure. It is therefore extremely difficult to control the filling pressure and the holding pressure, which possibly results in a crucial problem as to the quality of a molded article. In addition, there is a problem that the occurrence of a peak pressure shortens the life of the die and a clamping force necessary for the injection molding machine is increased.